Rectal motility and bioavailability.

The contractile activity of the canine rectal wall exhibits a positive influence on the behaviour of fatty suppositories in vivo with respect to both spreading abilities and rate and extent of release of the readily water-soluble compound phenazone. This influence on bio-availability was marked when the drug was suspended in a large particle size (100-125 µm). When used in small particles (< 35µm), far less influence of contractile activity was found. Small particles were equivalent to coarse particles with respect to the bioavailability. The addition of colloidal silicium oxide has a marked influence on spreading and bioavailability. Enhanced rectal motility exhibits an influence on the absorption only when a coarse fraction of the drug is suspended. It was concluded that rectal motility might be a cause of variation in bioavailability of drugs from rectal suppositories. For this reason only well-trained animals should be used when bioavailability of drugs from suppositories is tested in an animal model.